1. Field of the Invention
The present invention relates to a photoelectric conversion element equipped with a lower electrode, an upper electrode facing the lower electrode, and an organic photoelectric conversion layer formed between the lower electrode and the upper electrode, and to a solid-state imaging device.
2. Description of Related Art
Photo sensors are generally devices prepared by forming a photodiode (PD) within a semiconductor substrate such as silicon (Si) and, as a solid-state imaging device, a plane type solid-state imaging device is being widely used wherein PDs are two-dimensionally arranged within a semiconductor substrate, and signals each corresponding to a signal charge having been generated in each PD by photoelectric conversion are read out by CCD or CMOS circuit. As a technique for realizing a color solid-state imaging device, a structure wherein color filters each transmitting only a light of a particular wavelength are arranged for color separation on the light-incident side of the plane type solid-state imaging device is general. In particular, as a system at present widely used in, for example digital cameras, a single plate solid-state imaging device is well known wherein color filters each capable of transmitting blue (B) light, green (G) light, or red (R) light are regularly arranged on the two-dimensionally arranged individual PDs.
However, in the single plate solid-state imaging device, each of the color filters transmits only light of a particular wavelength, and light not transmitting through the color filter is not utilized, thus light-utilizing efficiency being bad. Also, with the increase in degree of integration, the size of PD becomes about the same size as the wavelength of light, and it becomes difficult for light to be wave-guided to PD. Also, since color reproduction is conducted by detecting blue light, green light, and red light by means of individual neighboring PDs and calculation-processing the data, there can result formation of a false color. In order to avoid this formation of false color, an optical low-pass filter is required, and there arises light loss due to the filter.
As a device for solving this problem, there has been reported a color sensor wherein three PDs are stacked within a silicon substrate utilizing the dependency of the absorption coefficient of silicon on the wavelength, and color separation is performed based on depth difference of the pn junction plane of each PD. In this system, however, there is involved the problem that the dependency on wavelength of spectral sensitivity with each of the stacked PDs is so broad that there results insufficient color separation. In particular, color separation between blue color and green color is insufficient.
In order to solve this problem, there has been proposed a stacked-type imaging device wherein an organic photoelectric conversion element capable of detecting green light to generate a signal charge in proportion to the detected green light is provided on or above a silicon substrate, whereas two PDs stacked within the silicon substrate detect blue light and red light, respectively. The organic photoelectric conversion element provided on or above the silicon substrate comprises a lower electrode stacked on the silicon substrate, an organic photoelectric conversion layer composed of an organic material and stacked on the lower electrode, and an upper electrode stacked on the organic photoelectric conversion layer and is constituted so that, when a voltage is applied across the lower electrode and the upper electrode, the signal charge having been generated within the organic photoelectric conversion layer moves to the lower electrode or to the upper electrode, and that a signal corresponding to the signal charge having moved to either of the electrode is read out by CCD or CMOS circuit provided within the silicon substrate.
With photoelectric conversion elements, there has been described a technique of stacking or mixing a fullerene in order to enhance photoelectric conversion efficiency. See, for example, JP-A-9-74216, JP-A-2004-165474, and JP-A-2007-123707.
However, with these photoelectric conversion elements, the photoelectric conversion efficiency is truly improved by the technique of stacking or mixing a fullerene for enhancing the photoelectric conversion efficiency, but the dark current is also increased, which results in the problem that a sufficient photo current/dark current ratio is unable to be obtained. This increase in dark current does not matter with organic solar cells, or the like, but is a fatal defect with such applications as organic imaging devices and organic image scanners which require a low dark current. Thus, it has been difficult to use the above-described conventional technique for such applications. In addition, since the absorption spectrum of a fullerene is so broad that, in the point of not only the dark current but absorption spectrum as well, it has been difficult to use a fullerene for the photoelectric conversion layer of an imaging device or a visible light-transmitting photoelectric conversion element having photoelectric conversion sensitivity to infrared region.
With organic photoelectric conversion elements wherein fullerene is used in the photoelectric conversion layer, optimization of the element structure has been conducted for the purpose of maximally enhancing the efficiency. For example, it has been reported that, with respect to the structure of photoelectric conversion layer, the structure wherein the volume ratio of an n-type organic semiconductor of fullerene to a p-type organic semiconductor of copper phthalocyanine is about 1:1 provides the highest efficiency as a solar cell (for example, Appl. Phys. Lett., Vol. 84, p 4218). However, most of the conventional investigations relate to solar cells, and there have been no reports on improvement of the photo current/dark current ratio which is necessary for a imaging device.
Also, with solar cells, a structure is required wherein the short-circuit current value in a non-biased situation upon irradiation with light, the leak current value upon application of bias, and a fill factor are maximized. However, dark current other than leak current does not particularly cause problems. For example, a dark current of about several μA/cm2 may be acceptable. On the other hand, imaging devices and image scanners are required to provide a large photo current and a small dark current upon application of bias. For example, in the case of photographing in the dark room, the photo current is at such a low level that the imaging device is required to provide a much lower dark current. Thus, it is required to suppress the dark current at a level of at most several nA/cm2 and, if possible, at a level of from about several hundred pA/cm2 to about several pA/cm2. For example, in the case of applying a bias of several V to an organic photoelectric conversion element having been optimized to the application of solar cells, the dark current is as large as several ten μA/cm2 though the efficiency is truly high, and hence the element is unable to be used as an imaging device. Also, in the case where the bias to be applied is reduced to a certain level or to zero in order to suppress the dark current, there results insufficient efficiency, thus the element being unable to be used as a imaging device.
Further, as is different from solar cells which are required to absorb visible light of wavelength region as broad as possible and conduct photoelectric conversion for taking out energy, imaging devices are required to have a sharp absorption spectrum. For example, they are required to have a sharp spectral sensitivity of about 100 nm in half-wave value and, for example, to absorb only a blue light having a peak in the range of from 400 nm to 500 nm, only a green light having a peak in the range of from 500 nm to 600 nm, only a red light having a peak in the range of from 600 nm to 700 nm, or only a near infrared light with transmitting the entire visible light. However, since fullerene has a wide absorption spectrum in the visible range, it is difficult to obtain a sharp spectral sensitivity necessary for an imaging device by forming a photoelectric conversion layer wherein fullerene and a p-type organic semiconductor are stacked with a volume ratio of about 1:1.